Radio receiver



Jan. 18, 1938; AAUNSgLL 2,105,789

RADIO -RECEIVER Filed May 16, 1953 A TToRNEY Patented Jan. 18, 1938hitaldt PATENT GEWEQE RADIO RECEIVER Alfred Aubyn Linsell, London,England, assignor to Radio Corporation of America, a corporation ofDelaware Application May 16, 1933, Serial No. 671,329

' In Great Britain May 25, 1932 Claims. (Cl. 250-20) This inventionrelates to radio receivers and more particularly to broadcast receiversand has for its object to provide a receiver which may be relativelyeasily and accurately tuned by comparatively unskilled persons to thewavelength of a desired transmitting station.

Many broadcast transmitting stations now in operation precede thetransmission of every programme by a transmission of a characteristicsignal modulation, this modulation being continued for a reasonable timeand being transmitted for the purpose of enabling the various personsdesirous of receiving the programme to tune their sets accurately to thestation in question. For example, the transmitting stations of theBritish Broadcasting Corporation emit a continucus audio frequencymodulation of substantially constant intensity and frequency for someminutes prior to the transmission of each programme, this audiofrequency modulation being generally referred to as the tuning note. Forthe purpose of convenience in definition a modulation such as that justdescribed which is transmitted for the purpose of facilitating thetuning of receivers will be referred to in the present specification asa characteristic tuning modulation. In practice receivers which aredesired to receive from a particular station emitting the characteristictuning modulation are tuned by their respective operators until the saidmodulation appears to be received in the best manner, but, moreparticularly in the case where a receiver is highly selective and/orincorporates re action means, this operation requires considerableskill. For example in the case of a set embodying reaction it is quiteeasy for an unskilled person to imagine that his set is accurately tunedwhen in fact the selective circuits therein are not accurately tuned butthe charactertistic tuning modulation is received at good strength byreason of excessive reaction or even self-oscillation, and accordingly,when the characteristic tuning modulation ceases and the programmecommences said programme will be heard in a distorted manner.

An object of the present invention is to provide a radio receiver withan indicator so arranged that an indication of correct tuning will begiven only when the characteristic tuning modulation is received notonly at good strength but in a relatively distortionless manner. Afurther feature of the invention resides in the fact that the indicationgiven is a visual indication and that, therefore, the difliculty atpresent met with, namely that of judging by the sound of thecharacteristic tuning modulation heard in the receiver Whether or notthe receiver is properly tuned, is avoided.

According to this invention a radio receiver is provided with a currentor voltage indicator, 5 a device or circuit arrangement responsive tothe characteristic nature of the characteristic tuning modulation foractuating said indicator, and means for energizing said responsivedevice or circuit arrangement from the demodulated signal currentobtained in the receiver.

The responsive device or circuit arrangement may take any of a varietyof forms and may be constituted by an electrical, electro-mechanical ormechanical coupling arrangement having sharply resonant characteristicsat the frequency of the characteristic tuning modulation, e. where, asis usual, this tuning modulation is a simple continuous single frequencyaudio medulation the responsive device or circuit arrange- 2 ment may bea tuned circuit or a tuning fork or similar resonant device whosefrequency is the same as that of the characteristic tuning modulation.

The invention is illustrated in the accompany- 5 ing diagrammaticdrawing wherein Fig. 1 shows an audio network of a receiver embodyingone form of the invention; and Figs. 2, 3 and 4 show, respectively,different modifications of the latter.

Referring to Figure 1 which shows one way of carrying out the inventionthere is arranged in cooperating relation with any convenienttransformer in the circuits of the receiver dealing with the demodulatedsignals 0. g. the ordinarily provided telephone transformer 2 leading tothe loudspeaker LS of the receiver, a tuning fork i which serves as theresponsive device. For example, the tuning fork may be arranged with onetine i a in the leakage field of the telephone or output transformer 2of the receive the other tine lb being arranged adjacent a coil 3 whichis in series with an indicating instrument 5 such, for example, as amilliammeter and a small rectifier 4, for example, a copper oxiderectifier.

Obviously the leakage field of any other audio frequency transformer orcoupling choke in the receiver may be similarly utilized but use of thefield of the ouput choke or transformer is preferred since ordinarilythis will be most suitable. The frequency of the tuning fork is the sameas that of the characteristic tuning modulation and it will be seentherefore that with this arrangement maximum indication will be given bythe milliammeter only when the set is properly tuned in such manner asto cause the demodulated signals corresponding to the characteristictuning modulation and flowing through the output transformer .to besubstantially pure and undistorted. If, however, the set is not properlytuned but is adjusted to give large volume merely by the use ofexcessive reaction the characteristic tuning modulation signals will notbe pure in form and. in consequence the response of the tuning fork willbe reduced. Means may be provided for cutting the tuning fork out ofaction when the receiver is in use for receiving an ordinary programme;for example, a mechanical gripping or damping device (not shown) may beassociated with the tines of the fork to prevent vibration and/or thefork may be placed in a sound proof box (not shown). In a slightlymodified arrangement shown in Figure 2 the leakage field of the outputtransformer 2 is not employed for driving the fork l the said fork beinginstead driven by a specially provided coil in series with the primaryof the output transformer 2. In this arrangement it is preferable toprovide a push button or other switch SW connected across the coil 8 sothat the said coil may be short circuited when the receiver is inordinary use. In a still further modification illustrated in Figure 3 aseries circuit consisting of the rectifier i and the milliammeter 5 isconnected across a specially provided coil 6 in series with the primaryof the output transformer 2 and the tuning fork I is arranged with onetine la adjacent this specially provided coil, i. e. the rectifier andmilliammeter are no longer in a coupled secondary circuit. As beforethis specially provided coil may be arranged to be short circuited orotherwise cut out of circuit e. g. by the switch SW when the tuningoperation is finished.

In place of using a tuning fork as the responsive device any otherconvenient resonant device or circuit arrangement may be employed; forexample as shown in Figure 4 a tuned circuit i of low damping tuned tothe frequency of the characteristic tuning modulation may be employed inplace of the tuning fork, the output from this circuit (which includesthe secondary of a transformer 6) being rectified and utilized toactuate the indicator. As before this tuned circuit may be arranged tobe short circuited or otherwise cut out of circuit by the switch SW whenthe tuning operation is finished. Again, in place of a tuning fork, areed or other mechanical or electro-mechanical vibratory resonant memberof low damping may be employed as the responsive device.

It will be seen that arrangements in accordance with this inventionenable tuning to be accomplished in a very simple and satisfactorymanner, the low frequency output from the receiver being utilized togive an indication to the operator of the set as to when the highfrequency tuning elements of that set are accurately adjusted. Ingeneral, in operating a set embodying the present invention the highfrequency tuning circuit or circuits, or the equivalent selective meanspreceding the demodulating detector, should be adjusted as closely aspossible to the wave length to be received using as little reaction aspossible, and then, if the volume is insufficient, the amount ofreaction should be increased up to a point preceding that at which theindicator, after reaching a maximum indica tion, begins to reduce itsindication due to a pure note no longer being received.

I claim:

1. Radio receiving apparatus including an au dio frequency circuit, atransformer in said circuit said transformer having primary andsecondary windings, a loudspeaker adapted to be energized from saidsecondary winding, a tuning fork having one tine adjacent saidtransformer and actuated thereby, a circuit having a reactance elementlocated adjacent the other tine of said tuning fork and actuatedthereby, and an indicating device in said second circuit.

2. Radio receiving apparatus comprising an audio frequency circuit, atransformer having its primary winding in said circuit, a loudspeakerconnected to the secondary winding of said transformer, a second windingin said audio frequency circuit adapted to be energized by acharacteristic tuning audio modulation current when the latter appearsin the audio circuit, means for short-circuiting said second windingwhen said apparatus has been tuned to develop the maximum tuning currentin the said second winding, a tuning fork actuated from said secondwinding and sharply resonant to the frequency of said tuning current, acircuit actuated from said tuning fork to develop a flow of current whensaid fork is actuated by said tuning current, and indicating means inthe last mentioned circuit.

3. Radio receiving apparatus including an audio frequency circuit, aloudspeaker and a transformer connecting said loudspeaker with saidaudio frequency circuit, a second transformer, connections connectingthe primary winding of said second transformer in said audio frequencycircuit, means operable to short circuit the primary winding of saidsecond transformer, a circuit connected to the secondary winding of saidtransformer, said circuit including a condenser connected in parallelwith said secondary winding to form a tuned circuit resonant at apredetermined frequency, and an indicating meter in the last mentionedcircuit.

4. In a receiving system of the type adapted to receive a characteristictuning modulation in the form of a continuous audio frequency modulationprior to normal intelligence modulated carrier waves, an audio frequencycircuit wherein the said tuning modulation currents flow, a visualindicating network for facilitating accurate tuning of the system, saidnetwork comprising a rectifier connected in circuit with an indicator,and a highly selective device, sharply resonant solely to said tuningmodulation frequency, electrically associated with the indicatingnetwork and'audio circuit and causing maximum current fiow in theindicating network when the receiver has been tuned to develop themaximum tuning modulation current in said audio circuit.

5. In a receiving system of the type adapted to receive a characteristictuning modulation in the form of a continuous audio frequency modulationprior to normal intelligence modulated carrier waves, an audio frequencycircuit wherein the said tuning modulation currents flow, a visualindicating network for facilitating accurate tuning of the system, saidnetwork comprising a rectifier connected in circuit with an indicator,and a highly selective device, sharply resonant solely to said tuningmodulation frequency, electrically associated with the indicatingnetwork and audio circuit and causing maximum current fiow in theindicating network when the receiver has been tuned to develop themaximum tuning modulation current in said audio circuit, and means forrendering the indicating network inoperative.

ALFRED AUBYN LINSEIL.

